1. Field of the Invention
This invention relates generally to the field of integrated circuit design and, more particularly, to the design of temperature sensor and measurement devices.
2. Description of the Related Art
Many digital systems, especially those that include high-performance, high-speed circuits, are prone to operational variances due to temperature effects. Devices that monitor temperature and voltage are often included as part of such systems in order to maintain the integrity of the system components. Personal computers (PC), signal processors and high-speed graphics adapters, among others, typically benefit from such temperature monitoring circuits. For example, a central processor unit (CPU) that typically “runs hot” as its operating temperature reaches high levels may require a temperature sensor in the PC to insure that it doesn't malfunction or break due to thermal problems.
Often, integrated circuit (IC) solutions designed to measure temperature in a system will monitor the voltage across one or more PN-junctions, for example a diode or multiple diodes at different current densities to extract a temperature value. This method generally involves amplifying (or gaining up) a small voltage generated on the diode(s), and then subtracting voltage from the amplified temperature-dependent voltage in order to center the amplified (gained) value for conversion by an analog-to-digital converter (ADC). In other words, temperature-to-digital conversion for IC-based temperature measuring solutions is often accomplished by measuring a difference in voltage across the terminals of typically identical diodes when different current densities are forced through the PN junctions of the diodes. The resulting change (ΔVBE) in the base-emitter voltage (VBE) between the diodes is generally proportional to temperature. (It should be noted that while VBE generally refers to a voltage across the base-emitter junction of a diode-connected transistor and not a voltage across a simple PN-junction diode, for the sake of simplicity, VBE is used herein to refer to the voltage developed across a PN-junction in general.) More specifically, a relationship between VBE and temperature is defined by the equation
                              V          BE                =                  η          ⁢                      kT            q                    ⁢          ln          ⁢                                    I              C                                      I              S                                                          (        1        )            where η is the ideality factor of the PN junction, k is Boltzman's constant, q is the charge of a single electron, T represents absolute temperature, Is represents saturation current and IC represents the collector current. A more efficient and precise method of obtaining ΔVBE is to supply the PN junction of a single diode with two separate and different currents in a predetermined ratio. Consequently, ΔVBE may be related to temperature by the equation
                              Δ          ⁢                                          ⁢                      V            BE                          =                  η          ⁢                      kT            q                    ⁢                      ln            ⁡                          (              N              )                                                          (        2        )            where N is a constant representing a pre-selected ratio of the two separate collector currents that are supplied to the PN junction of the diode.
In certain cases, for example when measuring the temperature of a semiconductor device such as a CPU, the PN-junction used in performing the temperature measurement may be comprised in a PNP device configured on the same substrate as the CPU. When using a small geometry process substrate transistor as the PNP device, the β (common-emitter current gain) of the transistor may be very low and may vary over process and temperature, as well as over collector current levels. Typical present day temperature measurement systems operate by applying controlled, ratioed currents to the emitter of a transistor used as the temperature measurement PNP device, and are therefore prone to temperature measurement errors due to the β variation in the transistor.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.